1. Field of the Invention
The present invention relates to a data transmission system in which a plurality of control terminals (nodes) used in plant control are connected to each other through a known LAN (Local Area Network) such as an ETHERNET (used herein to designate a LAN according to the IEEE 802 standard, e.g., IEEE 802.12 or 802.3 u), data exchange between the control nodes by using repeaters and management software arranged on the LAN.
2. Description of the Related Art
In recent years, in the field of plant control, control components such as personal computers and work stations which are not manufactured for industrial control have been used. When a plant control system is constructed by incorporating general personal computers and the like, an ETHERNET which is one of market standards which are frequently used as LANs in the field of OA (Office Automation), known application software for personal computers, TCP/IP (Transmission Control Protocol/Internet Protocol), and the like are desirably used.
In this case, since the ETHERNET employs a CSMA/CD (Carrier Sense Multiple Access/Collision Detection) scheme, a transmission apparatus is simplified, and transmission can be efficiently performed when traffic is small. However, when data collision occurs in the CSMA/CD scheme, a jam signal is sent to interrupt transmission. After waiting for a predetermined period of time defined by a random number, data is transmitted again. Even in this case, data collision occurs, so-called backoff algorithm control which performs waiting for a period of time defined by making the range of the random number twice is performed. For this reason, collision frequently occurs, long transmission time is required. As a result, even if the signal is sent again, transmission cannot be performed because of collision. In this manner, in the CSMA/CD scheme, collision occurs, after a waiting time defined by a stochastic method has elapsed, transmission is performed again. For this reason, when traffic increases, a waiting time required for re-transmission becomes long, and it becomes impossible to periodically transmit/receive data.
However, when data transmission is not reliably performed in plant control, a serious state such as plant stoppage may occur. For this reason, the plant control requires high reliability. Therefore, the ETHERNET cannot be easily used for data transmission in plant control. In fact, a scheme such as an MAP (Manufacturing Automation Protocol) which is a token bus scheme and can reliably periodically transmit/receive data has been often used.
In order to overcome the problem of the conventional ETHERNET to make it possible to use the ETHERNET, TCP/IP, and the like for data communication in a plant control system, the following means may be used. In the plant control system, PLCs (Programmable Logic Controllers) connected to nodes or various MMI (Man-Machine Interface) apparatuses, the same data is used in different control operations, different displays, different setting calculations, or the like. Therefore, when the ETHERNET is used as a data communication means in the plant control system, one-to-one communication is used to reliably transmit communication data, data request is performed at a time when the control data is required, and the control data must be respectively transmitted from nodes having the control data. In this case, when transmission is sequentially performed the number of destination nodes times with respect to one control data, the efficiency of use of communication resources is degraded.
There is provided an ETHERNET using a demand priority scheme in which no collision occurs with the data frame (data format) of the ETHERNET held. This ETHERNET using the demand priority scheme has a data transmission rate is increased to 100 Mbit/second (conventional ETHERNET: 10 Mbit/second). The Ethernet is being examined in IEEE to be used as a standard (IEEE 802.12: draft stage). The ETHERNET using the priority scheme is called a 100VG ANY LAN because the ETHERNET can cope with not only ETHERNET but also token ring. The ETHERNET is to be called as a demand priority scheme hereinafter.
In the demand priority scheme, a repeater connected to a plurality of nodes can process data having two types of priorities. When transmission requests having the same priority are received from the nodes, transmission rights is fairly given to the nodes in a round robin scheme. In this manner, control of the transmission requests is performed by the repeaters. A LAN in the demand priority scheme is basically constituted by repeaters and nodes, and the repeaters can have a plurality of hierarchies. The topology of the demand priority scheme is of a common bus type, and only one node has a transmission right at once as in the ETHERNET. The repeaters monitor transmission requests from the nodes and determines the next nodes when use of the transmission path is completed to permit transmission. The repeaters preferentially process a request having a high priority, and process a request a low priority later. A transmission right is given in the round robin scheme such that requests having the same priority fairly use the transmission path. The ETHERNET is a LAN of the market standards, and supports the TCP/IP serving as a protocol of the market standards. For this reason, a control LAN which can transmit control data and message data by using the demand priority scheme serving as a high-speed type ETHERNET is advantageously constituted.
However, when the ETHERNET having the demand priority scheme described above is employed as a data communication means for plant control, a problem in which a transmission disable time is extended by a backoff algorithm occurring in collision and which posed when a conventional ETHERNET using the CSMA/CD scheme is used can be avoided. However, the following problems are posed.
(1) Since processes are periodically performed in a programmable logic controller, when transmission requests for control data are sent in accordance with the interrupt cycle, the frequency of transmission requests is excessively high, the number of transmission requests exceeds the limited transmission capacity of the ETHERNET. PA1 (2) When transmission requests for control data are sent when the nodes request control data, the nodes must wait until other transmission performed in generation of transmission requests, the process of the programmable logic controller is delayed by the waiting time. PA1 (3) When the same control data is sent from the same node to a plurality of nodes many times, the ETHERNET cannot be efficiently used.